The reason why this does not work in general has already been pointed out in other answers: Whether or not the conversion is actually valid depends on the types of the objects that are contained in the original list. When there are objects in the list whose type is not of type TestB, but of a different subclass of TestA, then the cast is not valid.
Of course, the casts may be valid. You sometimes have information about the types that is not available for the compiler. In these cases, it is possible to cast the lists, but in general, it is not recommended:
... cast the whole list or
... cast all elements of the list
The implications of the first approach (which corresponds to the currently accepted answer) are subtle. It might seem to work properly at the first glance. But if there are wrong types in the input list, then a ClassCastException will be thrown, maybe at a completely different location in the code, and it may be hard to debug this and to find out where the wrong element slipped into the list. The worst problem is that someone might even add the invalid elements after the list has been casted, making debugging even more difficult.
The problem of debugging these spurious ClassCastExceptions can be alleviated with the Collections#checkedCollection family of methods.
A more type-safe way of converting from a List<Supertype> to a List<Subtype> is to actually filter the list, and create a new list that contains only elements that have certain type. There are some degrees of freedom for the implementation of such a method (e.g. regarding the treatment of null entries), but one possible implementation may look like this:
* Filter the given list, and create a new list that only contains
* the elements that are (subtypes) of the class c
* @param listA The input list
* @param c The class to filter for
* @return The filtered list
private static <T> List<T> filter(List<?> listA, Class<T> c)
List<T> listB = new ArrayList<T>();
for (Object a : listA)
This method can be used in order to filter arbitrary lists (not only with a given Subtype-Supertype relationship regarding the type parameters), as in this example:
// A list of type "List<Number>" that actually
// contains Integer, Double and Float values
List<Number> mixedNumbers =
new ArrayList<Number>(Arrays.asList(12, 3.4, 5.6f, 78));
// Filter the list, and create a list that contains
// only the Integer values:
List<Integer> integers = filter(mixedNumbers, Integer.class);
System.out.println(integers); // Prints [12, 78]